1. Field of the Invention
The present invention relates to a storage element including a storage layer for storing a magnetization state of a ferromagnetic layer as information, and a pinned magnetization layer with its magnetization direction fixed, wherein the direction of magnetization of the storage layer is changed through injection of spin polarized electrons therein by passing a current in a direction perpendicular to the film plane, and to a memory including the storage elements. The present invention is favorably applicable to nonvolatile memories.
2. Description of the Related Art
In information apparatuses such as computers, high-operating-speed and high-density DRAMs are widely used as random access memories (RAMS).
However, the DRAMs are volatile memories which loose information when the power supply is turned off and, therefore, nonvolatile memories which do not loose information even upon turning-off of the power supply are desired.
As a candidate for a nonvolatile memory, the magnetic random access memory (MRAM) operative to record information by use of magnetization of a magnetic material has been paid attention to, and development thereof has been progressing (refer to, for example, Nikkei Electronics, 2001 Feb. 12, pp. 164 to 171).
The MRAM is a memory in which electric currents are passed through two kinds of address wirings (word lines and bit lines) substantially orthogonally intersecting each other, and the magnetization of a magnetic layer of magnetic storage elements located at intersections between the address wirings is reversed by current-induced magnetic fields generated from the address wirings, thereby recording information.
Besides, reading of the information is achieved by use of the so-called magnetoresistance effect (MR effect) in which resistance varies according to the direction of magnetization of the storage layer in the magnetic storage elements.
A schematic diagram (perspective view) of a general MRAM is shown in FIG. 17.
A drain region 108, a source region 107, and a gate electrode 101 which constitute a selection transistor for selecting a memory cell are formed at a portion, isolated by an element isolation layer 102, of a semiconductor substrate 110 such as a silicon substrate.
In addition, a word line 105 extending in the front-rear direction in the figure is provided on the upper side of the gate electrode 101.
The drain region 108 is formed in common for the selection transistors arranged on the left and right sides in the figure, and a wiring 109 is connected to the drain region 108.
A magnetic storage element 103 having a storage layer of which the direction of magnetization can be reversed is disposed between the word line 105 and a bit line 106 located on the upper side and extending in the left-right direction in the figure. The magnetic storage element 103 is composed, for example, of a magnetic tunnel junction element (MTJ element).
Further, the magnetic storage element 103 is electrically connected to the source region 107 through a bypass line 111 extending in a horizontal direction and a contact layer 104 extending in the vertical direction.
When currents are passed through the word line 105 and the bit line 106, current-induced magnetic fields are applied to the magnetic storage element 103, with the result of reversal of the magnetization direction of the storage layer in the magnetic storage element 103, whereby information can be recorded.
In a magnetic memory such as an MRAM, for stable holding of the information recorded therein, it may be necessary that the magnetic layer (storage layer) for recording information has a fixed coercive force.
On the other hand, for rewriting the recorded information, it may be necessary to pass currents on a certain level in the address wirings.
However, attendant on the miniaturization of the elements constituting the MRAM, the address wirings are reduced in sectional size, so that it becomes difficult to pass sufficient currents in the address wirings.
In view of this, as a configuration capable of magnetization reversal by smaller currents, a memory designed to utilize magnetization reversal by spin injection has been attracting attention (refer to, for example, Japanese Patent Laid-open No. 2003-17782).
The magnetization reversal by spin injection means a process in which electrons having undergone spin polarization by passing through a magnetic material are injected into another magnetic material to thereby cause magnetization reversal in the another magnetic material.
For example, by a process in which a current is passed in a giant magnetoresistance effect element (GMR element) or a magnetic tunnel junction element (MTJ element) in the direction perpendicular to the film plane of the element, it is possible to reverse the magnetization direction of a magnetic layer at at least a part of the element.
In addition, the magnetization reversal by spin injection is advantageous in that the magnetization reversal can be realized with small currents even when the elements are miniaturized.
Schematic diagrams of a memory configured to utilize the magnetization reversal by spin injection as above-mentioned are shown in FIGS. 15 and 16. FIG. 15 is a perspective view and FIG. 16 is a sectional view.
A drain region 58, a source region 57, and a gate electrode 51 which constitute a selection transistor for selecting a memory cell are formed at a portion, isolated by an element isolation layer 52, of a semiconductor substrate 60 such as a silicon substrate. Of these components, the gate electrode 51 functions also as a word line extending in the front-rear direction in FIG. 15.
The drain region 58 is formed in common for selection transistors arranged on the left and right sides in FIG. 15, and a wiring 59 is connected to the drain region 58.
A storage element 53 having a storage layer of which the direction of magnetization can be reversed by spin injection is disposed between the source region 57 and a bit line 56 disposed on the upper side and extending in the left-right direction in FIG. 15.
The storage element 53 is composed, for example, of a magnetic tunnel junction element (MTJ element). Symbols 61 and 62 in the figure denote magnetic layers; of the two magnetic layers 61 and 62, one is a pinned magnetization layer of which the magnetization direction is fixed, while the other is a free magnetization layer, or storage layer, of which the magnetization direction can be changed.
In addition, the storage element 53 is connected to the bit line 56 and the source region 57 through upper and lower contact layers 54, respectively. This ensures that the magnetization direction of the storage layer in the storage element 53 can be reversed through spin injection by passing a current in the storage element 53.
As compared with the general MRAM shown in FIG. 17, the memory designed to utilize the magnetization reversal by spin injection as just-mentioned has also the characteristic feature that the device structure (element structure) can be simplified.
In addition, as compared with the general MRAM in which magnetization reversal is conducted by use of an external magnetic field, the memory utilizing the magnetization reversal by spin injection is advantageous in that the write current is not increased even upon a progress in miniaturization of the elements.
Meanwhile, in the case of the MRAM, the write wirings (word line and bit line) are provided separately from the storage element, and information is written (recorded) by use of the current-induced magnetic field generated by passing currents through the write wirings. Therefore, the currents necessary for writing can be sufficiently passed in the write wirings.
On the other hand, in the memory configured to utilize the magnetization reversal by spin injection, it may be necessary to reverse the magnetization direction of the storage layer by spin injection effected by the current passed in the storage element.
Since the writing (recording) of information is carried out by passing a current or currents directly in the storage element, a memory cell is configured by connecting the storage element to the selection transistor, for selecting the memory cell in which to write the information. In this case, the current flowing in the storage element is limited by the quantity of the current which can be passed through the selection transistor (the saturation current of the selection transistor).
Therefore, it may be necessary to write information with a current smaller of not more than the saturation current of the selection transistor and, hence, to reduce the current to be passed in the storage element by improving the efficiency of spin injection.
Besides, for enlarging the read current, it may be necessary to secure a high magnetoresistance variation ratio; for this purpose, a storage element configuration in which intermediate layers in contact with both sides of the storage layer are tunnel insulation layers (tunnel barrier layers) is effective.
In the case where the tunnel insulation layer is thus used as the intermediate layer, the current passed in the storage element is limited, due to the need to prevent dielectric breakdown of the tunnel insulation layer. From this viewpoint also, it may be necessary to suppress the current at the time of spin injection.
Therefore, in the storage element configured to reverse the magnetization direction of the storage layer by spin injection, it may be necessary to improve the spin injection efficiency and thereby to reduce the current needed for writing information.